1. Field of the Invention
The present invention relates to a steel sheet used for making three-piece cans and a method of making the steel sheet, and more specifically, to a method of manufacturing a thin and high strength can-making steel sheet excellent in flange formability and weldability.
2. Description of the Related Art
Can-making steel sheets are required to have a thin thickness-and high strength for reducing the cost of making cans. In addition to the above, three-piece can-making steel sheets are further required to be excellent in high speed weldability and flange formability after welding. This occurs because of the following reasons: When three-piece cans are made, can barrels used to form the cans are made by high speed electric welding at a speed up to 70 m/min. Moreover, after the can barrels have been made, a flange forming step is carried out on the can barrels such that each barrel contains a welded portion to mount upper and bottom lids.
However, the prior art has had the problem that when the thickness of a sheet is reduced, the proper welding range is narrowed. Thus, when the welding current is increased, splashing is caused, the weld is hardened and flange cracking tends to occur at the HAZ (heat affected zone) of the weld in a formed flange after formation of a can barrel.
Further, in a recent can-making process, a coil-coated steel strip, which was previously coated at the stage of a coil or a film-laminated steel strip which is made by bonding a printed film to a steel sheet coil, was employed to improve the efficiency of the coating process. In those methods, so-called non-varnished portions to which no coating has been applied or no film has been laminated are preferably formed parallel to the rolling direction when a can is made in order to effectively carry out slitting. However, problems arise in that when the non-varnished portion of a sheet prepared as described above is welded, and then flange forming is carried out, cracking is liable to occur at the HAZ. To cope with this problem, the non-varnished portions are conventionally formed in a direction perpendicular to the rolling direction. Thus, a coil-coated steel strip and a film-laminated steel strip cannot be effectively manufactured.
Many efforts have been made to solve these problems. However, no proposals for directly solving these problems has been found for can-making steel sheets. For example, although Japanese Patent Examined Publication No. 1-52450 discloses a method of manufacturing a can-making steel sheet T1-T3 by continuously annealing ultra-low carbon steel and then temper-rolling the same, it does not disclose a solution to the above problems.
Although U.S. Pat. No. 4,889,566 discloses an ultra-low carbon steel sheet added with Ti, Nb, B to improve spot weldability of automobile-making steel sheets, the patent does not provide any suggestion with respect to problems characteristic to three-piece can-making steel sheets.
Although U.S. Pat. No. 5,156,694 discloses a method of adding Nb, B and further Ti to an ultra-low carbon steel to improve press formability, deep drawability and further fatigue resistant strength at a weld of an automobile-making steel sheet, it mentions nothing with respect to the welding characteristics of can-making steel sheets. Further, the patent mentions nothing with respect to the formability of the HAZ portion of a weld.